Various types of prime movers are used in current production automobiles. Prime movers are machines, such as motors and engines, in which work is accomplished by the conversion of various forms of energy into mechanical force and motion. For example, motors convert electric energy into mechanical energy, and engines transform chemical energy into mechanical energy. Such vehicle engines are sized to meet relatively high load requirements, such as hill climbing or rapid acceleration from standstill. But during normal city and highway vehicle operation at cruising speeds, a vehicle engine is often operated according to relatively low loads. Accordingly, vehicle engines usually operate at excess capacity, which can result in relatively low engine efficiency and high fuel consumption.
Several approaches have been proposed to modify a vehicle powertrain to yield higher engine efficiency and lower fuel consumption, including the use of multiple separate engines coupled to a conventional automatic transmission. When such a vehicle is operated at a relatively low torque demand from the operator, only one of the engines is employed to propel the vehicle. When torque demand exceeds the maximum torque output of the single engine, a second engine is activated in synchronism with the first engine and is engaged to the transmission to supplement the first engine torque output to the transmission.
But several drawbacks have been noted with this arrangement. For example, existing engine output couplings are typically not highly reliable, usually do not provide smooth operation transparent to the vehicle operator, and are often relatively expensive. Furthermore, existing coupling mechanisms do not always enable the engines to be coupled relatively quickly, and may involve substantial lag time between the vehicle operator's demand for additional torque and the delivery of additional torque via the second engine. Finally, in a powertrain with multiple engines, the initializing, activating, and/or synchronizing of additional engines is relatively fuel inefficient.